1. Field of the Invention
The present invention relates to a surface acoustic wave element used in telecommunication devices, and in particular relates to a surface acoustic wave element that uses a piezoelectric thin film.
2. Description of the Related Art
There has been a remarkable expansion in the demand for surface acoustic wave elements with the rapid developments in the telecommunication field centered on mobile communication, which is typified by mobile telephones. Some trends in the development of surface acoustic wave elements include size reduction, increasing efficiency, and increasingly higher frequencies. In order to attain these, a larger electromechanical coupling coefficient (below, k2), more stable temperature characteristics, and a higher surface acoustic wave propagation velocity become necessary. For example, when used as a high frequency filter, a high k2 is desirable in order to obtain a pass band having a small loss and a wide bandwidth. In order to make the resonance frequency a high frequency, a material having a faster acoustic velocity is desirable in view of the limits of the design rules for the pitch of interdigital transducers (below IDT). Furthermore, in order to stabilize the characteristics of the temperature range in which surface acoustic wave elements are used, the center frequency temperature coefficient (TCF) must be small.
Conventionally, surface acoustic wave elements generally have a structure in which an IDT is formed on a single crystal piezoelectric body. Representative piezoelectric single crystals are quartz, lithium niobate (below, LiNbO3), lithium tantalite (below, LiTaO3), and the like.
For example, in an RF filter requiring a broad band and low loss in the pass band, LiNbO3, which has a large k2, is used. In contrast, in an IF filter requiring stable temperature characteristics even in a narrow band, quartz, which has a small TCF, is used. Furthermore, LiTaO3 plays an intermediate role because its k2 and TCF are each between those of LiNbO3 and quartz. However, even for LiTaO3, which has the highest k2, the k2 is about 20%.
Recently, a cut angle that exhibits a large k2 value has been discovered in the KNbO3 single crystal (a=0.5695 nm, b=0.5721 nm, c=0.3973 nm; below, the orthorhombic crystal is represented by these indices). As reported in Electron. Lett. Vol. 33 (1997) 193, it can be predicted by calculation that a 0° Y-cut X-propagation (below, 0° Y-X) KNbO3single crystal plate shows an extremely high value of k2=53%. Furthermore, as reported in Jpn. J. Appl. Phys. Vol. 37 (1998) 2929, it has been experimentally confirmed that a 0° Y-X KNbO3 single crystal plate demonstrates a high value of k2 (about 50%), and it is reported that the oscillation frequency of the filter using the Y-X KNbO3 single crystal plate rotated from 45° to 75° demonstrates zero temperature properties at room temperature. A patent application has been filed for these rotated Y-X KNbO3 single crystal plates that includes 0° Y-X as Japanese Unexamined Patent Application, First Publication, No. Hei 10-65488.
In surface acoustic wave elements that use a piezoelectric single crystal substrate, characteristics such as k2, the temperature coefficient, sound velocity and the like are values intrinsic to the material, and are determined by the cut angle and the propagation direction. A 0° Y-X KNbO3 single crystal substrate has a superior k2, but the zero temperature properties like those of the Y-X KNbO3 single crystal substrate rotated from 45° to 75° are not exhibited at room temperature. In addition, the propagation velocity is slow in comparison to SrTiO3 and CaTiO3, which are also perovskite-type oxides. Thus, when only a KNbO3 single crystal substrate is used, the sound velocity, high k2, and zero temperature properties cannot all be satisfied.
Thus, a piezoelectric thin film is laminated on some sort of substrate, film thickness is controlled, and thereby it is anticipated that the sound velocity, k2, and temperature characteristics will be improved. Examples include a zinc oxide (below, ZnO) thin film formed on a sapphire substrate, as reported in Jpn. J. Appl. Phys. Vol 32 (1993) 2337, or a LiNbO3 thin film formed on a sapphire substrate, as reported in Jpn. J. Appl. Phys. Vol. 32 (1993) L745. Therefore, for KNbO3 as well, it is anticipated that all properties will be improved by depositing a thin film onto a substrate.
Here, it is preferable that the piezoelectric thin film be oriented in an optimal direction in order to exhibit its k2 and temperature characteristics, and it is preferable that it be a flat, compact epitaxial film in order to minimize as much as possible the loss that accompanies leaky wave propagation. Here, a Y-X KNbO3 having a k2 of about 50% corresponds to the pseudo-cubic crystal (100), and the 90° Y-X KNbO3 having a k2 of 10% corresponds to the pseudo-cubic crystal (110). Therefore, for example, by using a SrTiO3(100) or (110) single crystal substrate, it is possible to obtain a Y-X KNbO3 thin film having a k2 of about 50% or a 90° Y-X KNbO3 thin film having a k2 of about 10%.
However, the surface acoustic wave element using these KNbO3 piezoelectric thin films have the following problems.
When manufacturing a KNbO3 thin film, K vaporizes easily in comparison to Nb because the saturation vapor pressure of K is extremely high compared to Nb. Thus, there is the problem that in comparison to the starting composition, the composition of the thin film after manufacture deviates towards the Nb excess side. In order to compensate for this deviation in composition, as reported, for example, in Appl. Phys. Lett. Vol. 68 (1996) 1488, when manufacturing a KNbO3 thin film by using a laser ablation, a target made in advance with an excess of K is used. However, as is clear from the phase diagram of the K2O.Nb2O5 shown in FIG. 1 of J. Am. Chem. Soc. Vol. 77 (1955) 2117, on side where the composition of the KNbO3 has a K excess, a 3K2O.Nb2O5 compound is present. At or below the eutectic temperature of 845° for KNbO3 and 3K2O.Nb2O5, both KNbO3 and 3K2O.Nb2O5 coexist as a solid phase. On the side where the composition of the KNbO3 has an Nb excess, a 2K2O.3Nb2O5 compound is present, and at or below the melting point of 1039° C. of KNbO3, both the KNbO3 and the 2K2O.3Nb2O5 coexist in a solid phase. Therefore, at the point in time that the starting material that has been ablated by a laser arrives at the substrate, if the composition is not strictly K:Nb=50:50, a film manufactured either on the K excess side or the Nb excess side will include different phases, and a single phase cannot be obtained.
Here, in the case of a KNbO3 bulk single crystal, a large single crystal can be obtained, for example, by pulling up the single crystal from a liquid phase (Top-Seeded Solution Growth (TSSG)) having a composition with a slight K excess over the K:Nb=50:50 by using a seed crystal. In the K2O.Nb2O5 two dimensional system phase diagram in FIG. 1, this is obtained by placing the starting material having a composition from K2O:Nb2O5=50:50 to about K2O:Nb2O5=65:35 within the range in which the KNbO3 and the liquid phase coexist. The KNbO3 is present at or above the eutectic temperature of 845° C. of KNbO3 and 3K2O.Nb2O5. That is, in FIG. 2, when the starting material having composition C1 is cooled from the liquidus temperature T11 to the crystal growth temperature T12, the KNbO3 precipitates from the liquid phase, and the liquid phase deviates towards the K excess side up to composition C2, in which T12 serves as the liquidus temperature. Because the crystal growth rate at this time becomes higher as C1–C2 becomes larger, preferably there is slightly more K than KNbO3, and as far as possible a composition close to KNbO3 is cooled down to the region of the eutectic temperature 845° of the KNbO3 and the 3K2O.Nb2O5. The above behavior occurs in the atmosphere, and furthermore, occurs when a KNbO3 bulk single crystal is grown from a high volume liquid phase.
Thus, it is an object of the present invention to provide a crystal growth process in which a single crystal is precipitated from the liquid phase in the atmosphere, that is applied to a thin film production process using a vapor phase method under reduced pressure, thereby growing epitaxially a high quality KNbO3 single crystal thin film on any type of single crystal substrate. Furthermore, it is an object of the invention to provide a surface acoustic wave element that can be adapted to high frequencies, has a high k2, and can be expected to exhibit effects with respect to size reduction and energy savings.